Aluminum chloride is an important chemical which has many industrial applications. For example, it is used as a catalyst in organic chemical syntheses and in petroleum refining. It is also used in dyestuffs and as a nucleating agent for titanium dioxide pigments. In addition, recent technological developments have signaled the potential need for large tonnages of aluminum chloride in processes for the production of aluminum metal. In such processes, an aluminous material is first chlorinated to provide AlCl.sub.3, after which the AlCl.sub.3 is purified and subsequently reduced by electrolytic or chemical means to give aluminum metal. Such a chemical reduction is effected, for instance, by the Toth Process in which manganese metal is employed to reduce aluminum chloride to give aluminum metal and manganese chloride. The manganese chloride is converted to manganese metal and chlorine which are recycled. The Toth Process is described in the following U.S. Pat. Nos. 3,615,359, 3,615,360; 3,677,742; 3,713,809 and 3,713,811.
In many of these uses of AlCl.sub.3, contamination of the AlCl.sub.3 with iron chloride and other chlorides is deleterious because of, for example, discoloration in dyeing, the variation of pigment reaction in catalysis and the presence of impurities in aluminum metal.
Current domestic production of AlCl.sub.3 is carried out exclusively through direct chlorination of metallic aluminum of high enough purity that purification of the AlCl.sub.3 is generally unnecessary. However, methods for production of AlCl.sub.3 which employ aluminum metal would obviously not be of use in processes for the production of aluminum. A more economical means for producing aluminum chloride from carbo-chlorination of an aluminous material would be required. Such a process would also be far more economical than production from aluminum, particularly if economical methods were available for purification of the AlCl.sub.3. Such materials for chlorination may be high purity but expensive alumina made by the Bayer Process. Alternately, an aluminous ore such as bauxite or clay may be employed, and these materials may be directly carbo-chlorinated following calcination. In the latter case, however, vapors produced during chlorination contain not only AlCl.sub.3 but also volatile chlorides of certain impurities commonly present in the ore, such as ferric chloride, titanium tetrachloride, and silicon tetrachloride. For example, a typical analysis of Georgia kaolin clay is given below and each of the listed metallic components would appear as chloride vapors in the product formed during the carbo-chlorination.
______________________________________ Component Weight Percent ______________________________________ Al.sub.2 O.sub.3 30.0 SiO.sub.2 50.0 Fe.sub.2 O.sub.3 3.0 TiO.sub.2 0.6 H.sub.2 O 15.0 Others 1.4 100.0 ______________________________________
At atmospheric pressure, AlCl.sub.3 condenses as a solid, so that conventional methods for obtaining aluminum chloride from mixed chloride vapors formed during the chlorination of calcined ores generally involve an initial condensation of the vapors directly to the solid form. Such condensation may be total condensation, but more likely it involves a fractional condensation of the less volatile aluminum chloride and ferric chloride. Titanium tetrachloride and silicon tetrachloride have significantly higher vapor pressures and consequently are readily separated by condensation at substantially lower temperatures than aluminum chloride and ferric chloride.
Commercially, the solid condensation of aluminum chloride and ferric chloride has been carried out in scraped-surface condensers such as those manufactured by Vogt Machine Co. and the Votator Division of Chemetron Corporation. An example of a commercial application of these condensers to solidify AlCl.sub.3 vapors formed during the chlorination of ore is the Alchlor Process of Gulf Refining Co. The process and the condensers are described in an article by A. M. McAfee, "The Manufacture of Commercial Anhydrous Aluminum Chloride", Ind. and Engr. Chem., Vol. 21, No. 7, page 670, and in U.S. Pat. No. 1,814,397. However, scraped-surface condensers are expensive pieces of equipment and require extensive maintenance. Thus capital and operating costs for this method are extremely high.
After solidification of the aluminum chloride and ferric chloride, purified aluminum chloride is obtained by melting the solids under pressure, and by subsequent distillation to provide aluminum chloride of the desired purity. This is also a costly operation because of the high heats of fusion and vaporization of aluminum chloride (116 and 100 BTU/pound respectively) and the inefficiency of melting a solid which has been condensed from the vapor state.
Another method of obtaining purified aluminum chloride is to reduce the ferric chloride contaminant to non-volatile ferrous chloride or iron metal using aluminum metal. The aluminum chloride is then volatilized and condensed. However, this method also requires the melting of the condensed solid aluminum chloride and ferric chloride plus the added expense of the aluminum metal required to react with the ferric chloride to form aluminum chloride and iron metal.
Other prior art methods have employed titanium tetrachloride to preferentially dissolve the solidified aluminum chloride, leaving the ferric chloride in suspension to be filtered or centrifuged away. Although the solubility of aluminum chloride in titanium tetrachloride is relatively low, ferric chloride solubility is negligible with the result that a good separation is obtained. However, large quantities of titanium tetrachloride are required, thus making the recovery of aluminum chloride expensive, especially since the solid condensation of AlCl.sub.3 is still necessary.
Another possibility which has not been tried commercially is to compress the mixed chloride vapors, condense these vapors directly to the liquid state, and then proceed with one or more of the previously mentioned methods to obtain purified aluminum chloride. However, because of the high temperatures involved during such compression, and the large scale of the operation that would be required, this would be a very expensive procedure.